A. Field of the Invention
This invention relates to methods for avoiding the agglomeration of pellets of several kinds of materials when treated at high temperatures and, more particularly, to a method for avoiding the agglomeration of pellets by covering the pellets with a long term-stabilized magnesium hydroxide suspension, having a stability of at least three months without substantially settling and without forming a hard solid substrate including a compound that improves the adhesion of the suspension to the pellets
B. Description of Related Art
In some treatments of bricks or pellets of materials such as dolomite, iron mineral, etc., in which it is necessary to sinter a plurality of bricks inside furnaces at high temperatures, there exists a tendency of said bricks or pellets to adhere to each other producing an agglomerate of bricks.
One of these treatments is the iron reduction of mineral iron powder and pellets inside a furnace at high temperatures. When the sintering is complete, usually some or all of the pellets are agglomerated in a solid mass which obstruct the furnace exit and makes necessary to apply a mechanical shock in order to separate each pellet from the agglomerate.
In order to avoid the agglomeration of the pellets during sintering, several compounds were tested which must be applied to the pellets surface before the sintering process. Among these compounds, there were tested slurries of magnesium hydroxide, calcium hydroxide, bauxite and even cement, but none of these compounds produced the expected results and had some disadvantages when used for the purpose referenced above.
Some of the principal disadvantages, in common with all the compounds previously mentioned, are that the compounds must be continuously stirred to maintain a constant concentration, and that once the compound was applied to the pellets, the compounds proved to have a very poor adherence to the pellets, since the cover, once it dries, tends to separate from the pellet simply by the movement produced by the normal manipulation of the pellets.
The magnesium, and particularly the magnesium hydroxide, has a great number of applications, such as a flame retardant, neutralization of acid waste streams, as well as for pharmacological purposes and detergent production.
In some production plants, the magnesium hydroxide is produced from a magnesium chloride brine and burnt dolomite, in order to obtain dead burned magnesium oxide, to be used mainly in the manufacture of refractory bricks.
Other methods for the production of magnesium hydroxide are:
Production of magnesium hydroxide by reacting dolomite or limestone with sea water or chloride or magnesium brines.
Production of magnesium hydroxide by thermal decomposition of chloride or magnesium brines.
Production of magnesium hydroxide by burning natural magnesite (magnesium carbonate) in order to obtain magnesium oxide which afterwards is hydrated with water in order to obtain magnesium hydroxide.
Production of magnesium hydroxide by hydrating low cost magnesium oxide (such as that which originates in China).
Production of magnesium hydroxide by reacting sodium hydroxide with magnesium sulfate or Epsom salt.
Production of magnesium hydroxide from sodium hydroxide and chloride and magnesium brines.
From ammonia or ammonium hydroxide with chloride or magnesium brines.
Looking for a suitable substance for covering the pellets and iron powder for avoiding its agglomeration when treated at high temperatures, trials were carried out for the production of magnesium hydroxide.
The first trials commenced by using a magnesium hydroxide paste obtained by: mixing burned dolomite and magnesium chloride brine in order to obtain a reaction between both compounds; washing the product of said reaction and conditioning the product in order to obtain a suspension having a high covering area per gram of substance (surface area covered by each gram of substance) and a viscosity of less that 100 centipoises, which would allow its being pumped by any kind of pumping means such as a centrifugal pump, a diaphragm pump or a peristaltic pump. Said suspension was subsequently conditioned by the addition of a compound that improves the adhesion of the suspension to the pellets by which the viscosity of the suspension was increased to approximately 1,000 centipoises.
When is necessary to send the magnesium hydroxide paste or suspension to locales remote from the production plant, as well as for further processing, it is necessary to store a magnesium hydroxide suspension for long terms in a storage container. Under such circumstances, the solid particles of the suspension normally tend to settle out producing a hard solid substrate as a hard cake in the bottom of the storage container, which is often very difficult to remove. The remaining suspension under such circumstances, results in a useless material since it doesn't have the required magnesium hydroxide concentration nor the required particle size in suspension and, therefore, it has to be poured from the storage container and discarded in order to remove the hard solid cake from the bottom of the container, resulting in increased costs and high raw material losses.
Therefore, it would be highly desirable to have a method for covering pellets of a variety of materials in order to avoid the agglomeration of a plurality of pellets when stored or subjected to a heat treatment thereof using s magnesium hydroxide suspension having good adhesive properties and the capability of being stored for long periods of time without substantial settling and cake formation.
In accordance with the present invention a method for avoiding the agglomeration of pellets of several kinds of materials when treated at high temperatures is provided, which comprises: covering the pellets with a a 50-60% magnesium hydroxide suspension, called “Femag HL” having a solid content between about 51% to 61%, a water content of from about 39% to 49%, a viscosity of about 1000 centipoises (cp)., a particle size of about 2 microns, a chloride content less than about 0.6% by weight, a calcium content of less than about 1%, a pH greater than about 10, an equivalent magnesium oxide content of about 34% to 42%, a specific gravity of about 1.42 to 1.52, and including one or more anionic polyelectrolytes as a dispersant agent, at a concentration of at least about 25%, in an amount of about 0.5 to 2.5% on a dry basis, and a compound that improves the adhesion of the suspension to the pellets developed specifically for the above referred purpose called “GBC200” comprising an acrylic-styrene emulsion, at a concentration of at least about 30% in an amount of about 0.5 to 5%, preferably about 1.5% to 2% on a dry basis, exhibiting a stability of at least three months without substantial agitation.
By covering the pellets with the magnesium hydroxide suspension as disclosed by the method of the present invention, there are obtained the following benefits:
Lower maintenance costs of conveyor belts, rollers, sieving devices, etc.
Easier product manipulation procedures.
Reduced emanation of hazardous and polluting elements to the environment. High quality iron pellets are obtained.
Benefits to the steel industry with respect to slag fluidity.